Recent studies have shown that a significant proportion of human tumors contain ras genes that have suffered genetic alterations resulting in the acquisition of potential to oncogenically transform normal cultured rodent fibroblasts. To date, three human ras genes have been characterized, c-H-ras, c-K-ras and N-ras, and the available evidence indicates that they have been activated by somatic mutation of their corresponding normal protooncogenes. The activation mechanisms of ras oncogenes have been established as point mutations in their protein coding region. At the same time, DNA transfection experiments have shown that overexpression of normal ras genes can also induce the oncogenic transformation of rodent established cell lines, ras genes have been found amplified in some animal and human tumors, and a correlation has been found between the level of expression of ras genes (normal or mutant) and the cellular phenotype. Thus, although the role that ras oncogenes could be playing in the origin, progression or maintenance of these tumors remains to be determined, it seems likely that activation of ras genes either by structural mutations and/or by abnormal expression could play an important role in the multistep process of human carcinogenesis. We have isolated the c-K-ras oncogene from human lung carcinomas and the structural features relevant to its mutational activation have been determined. We propose to analyse in detail the molecular mechanisms that control the regulated expression of the c-K-ras gene in human normal and tumor cells. The in vivo expression pattern of the gene will be studied by molecular hybridization and immunological techniques. The regulatory signals contained in the primary structure of the gene that control its expression at the transcriptional and/or posttranscriptional levels will be analyzed by in vitro manipulation of cloned oncogene sequences in concert with DNA mediated gene transfer experiments. These studies should provide information essential for the understanding at the molecular level of the cellular processes involved in the expression of this human gene which is closely associated with the development of a significant number of human cancers in particular, and of gene regulation in eukaryotes in general.